1. Field of the Invention
This invention relates generally to line scanning apparatus and, more particularly, to a helical line scanning apparatus especially compensated for beam divergence.
2. Description of the Prior Art
Line scanning apparatus of the type embodying a rotating pyramidal or polygonal mirror comprising individual reflecting surfaces to scan a beam across a target are well known in the art as mechanical flying spot scanners. Line scanning apparatus of a type disclosed in U.S. Pat. No. 3,523,160, entitled "Optical Scanning Device Having A Constant Optical Path Length", by R. Willey, issued Aug. 4, 1970, include a helical mirror from which an optical beam is reflected in a manner whereby one complete scan of a target is made for every revolution of the helix. The line scanning device may be utilized with either a noncollimated light beam or a collimated light beam, such as a laser beam. Willey concludes that the use of a collimated light source with his scanning device eliminates the focusing problem and permits the target to be placed at any desired distance from the drum and at practically any angle with respect to the reflected beam. However, depending upon the precision of the line scan required, focusing problems can materialize particularly in situations requiring a precise line scan to provide photographic quality prints, despite the use of a laser as the light source. Such problems arise as a result of the collimated light beam being reflected by the helical surface which by its very nature cannot be made to conform to a zero curvature. Hence, the reflected light beam from the helical surface is inherently astigmatic and thereby increases the width of the line scan and decreases the resolution to which each line scan can be modulated.
Therefore, it is a primary object of this invention to provide a line scanning apparatus which is especially compensated and masked in a simple and economical manner to minimize the effects of the divergence of a light beam reflected from a helical surface.
Other objects of the invention will in part be obvious and will in part appear hereinafter. The invention accordingly comprises the mechanism and system possessing the construction, the combination of elements and the arrangement of parts which are exemplified in the following detailed disclosure.